The default degree of the best least squares polynomial is 2. There are buttons to choose "linear", "quadratic", or "cubic". Or one can type any desired degree. The degree is limited to 0 <= degree <= number of points minus one.
A mouse click in the graph adds a point to the list. As many points can be added as one wants to click on. Points can be deleted by clicking on them. It can be sometimes hard to "hit the point" exactly and one ends up with two points close together. Dragging a zoom rectangle around the points magnifies them and makes it easy to click on one or the other to delete it.
One can delete all but one of the points. And there is a button to restore everything to the original data if desired.
When one has many points the polynomial can have a large range, hence it is useful to have a "Zoom Y" button to allow zooming out so as to see what the polynomial looks like when brought into scale. Of course there is a "restore zoom" button.